The recruitment of immune cells to sites of injury involves the concerted interactions of a large number of soluble mediators. Several cytokines appear to play roles in these processes, for example, IL-1 and TNF. Both cytokines are derived from mononuclear cells and macrophages along with other cell types. Physiologically, they produce many of the same proinflammatory responses, including fever, sleep, anorexia, mobilization and activation of polymorphonuclear leukocytes, induction of cyclooxygenase and lipoxygenase enzymes, increase in adhesion molecule expression, activation of B-cells, T-cells and natural killer cells, and stimulation of production of other cytokines. Other actions include a contribution to the tissue degeneration seen in chronic inflammatory conditions, such as stimulation of fibroblast proliferation, induction of collagenase, etc. They have also been implicated in the process of bone resorption and adipose tissue regulation. Thus, these cytokines play key roles in a large number of pathological conditions, including rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, diabetes, obesity, cancer, and sepsis.
The role of IL-1 in inflammation has been demonstrated by the ability of the highly specific IL-1 receptor antagonist protein (IL-1 Ra, or TRAP) to relieve inflammatory conditions. IL-1 treatment of cells induces the formation of a complex consisting of the two IL-1 receptor chains, IL-IRI and IL-1 RAcP, and the resulting heterodimer recruits an adaptor molecule designated as MyD88. MyD88 binds to a protein designated IRAK (Interleukin-1 Receptor-Associated Kinase). IRAK is subsequently phosphorylated and released from the receptor complex to interact with a tumor necrosis factor receptor-associated factor, TRAF6, which transduces the signal to downstream effector molecules. TRAF6 can trigger the NIK/IKK kinase cascade to activate the transcription factor NF-κB. NF-κB regulates a number of genes that, in turn, regulate immune and inflammatory responses.
Four IRAKs have been identified: IRAK-1, IRAK-2, the monomyeloic cell-specific IRAK-M, also known as IRAK-3, and IRAK-4. IRAK proteins have been shown to play a role in transducing signals other than those originating from IL-I receptors, including signals triggered by activation of IL-18 receptors and LPS receptors. Overexpression of IRAK-2 and IRAK-M has been shown to be capable of reconstituting the response to IL-1 and LPS in an IRAK deficient cell line.